Human blood coagulation factor XIII (hereinafter referred to briefly as factor XIII) is involved in the formation of a stable fibrin polymer in the final phase of the mechanism of blood coagulation. Known, also, as fibrin stabilizing factor, factor XIII is normally present in an inactive form in the blood but when a coagulation of blood following a bleeding or the like induces the formation of thrombin, it is activated under the influence of this thrombin and calcium ions to stabilize the fibrin. Therefore, in factor XIII-deficient blood, the coagulation time shows a value within the normal range but the fibrin clot formed is so fragile as to induce some characteristic phenomena such as after-bleeding. Therefore, the blood concentration of factor XIII gathered attention as a possible indicator of factor XIII-deficient patients. Today, reagent kits for neutralizing antibody assay and monochloroacetic acid dissolution assay are commercially available.
However, this immunoassay technique for factor XIII determination is so low in detection sensitivity, i.e. a dilution limit only down to 1 to 3% of the factor XIII concentration of healthy humans, that in laboratory tests, for instance the calibration of the factor XIII preparation or the screening test for factor XIII-deficient patients cannot be performed with adequate accuracy. It is, therefore, a problem awaiting a solution to establish an assay method which permits a rapid and accurate determination of factor XIII in a large number of samples.